Alloy and articles made therefrom



R. E. SAVIDGE 2,888,345

ALLOY AND ARTICLES MADE THEREFROM May 26, 1959 'Filed April 1, 195a lnvenfor- Roland E.Sc1vidge b3 His Arfbrneg United States Patent 1 2,888,345 ALLOY AND ARTICLES MADE THEREFROM Roland E. Savidge, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Application April 1, 1958, Serial No. 725,583 8 Claims. (Cl. 75128) This invention relates to new and useful alloys. More particularly, it relates to new alloys which are characterized by superior corrosion resistance, castability, good weldability, and other desirable mechanical properties.

In a reaction type steam turbine, the translational velocity of the steam is converted into rotational velocity for driving electrodynamic machinery such as turbine generators and the like by directing the steam through successive rings of partitions or fixed nozzles, each of which has a corresponding ring of rotatable blades or buckets, the nozzles and blades being properly disposed to attain the desired rotational movement. Generally speaking, the partitions or nozzles, being most directly exposed to the stream of flowing steam, are made of corrosion resistant steels, such as those containing relatively high quantities of chromium. In steam turbines using relatively dry steam, it has been found that the parts of the diaphragm in which the partitions are mounted, namely, the rim and the web or hub, canbe conveniently of low cost materials, such as plain carbon steel, gray cast iron and the like. While in some cases the diaphragms are of fabricated construction with the partitions welded in, it has been found convenient also to cast the partitions or nozzles into the web and rim of the diaphragm. While low-grade diaphragm materials, such as plain carbon steel and gray cast iron, are entirely adequate in most cases, as the water or moisture and oxygen content of the steam increases under certain operating conditions, it has been found that these lowgrade materials tend to be eroded by the droplets of the moisture moving in the high velocity stream of steam. At the same time, the relatively higher oxygen content tends to corrode excessively the lower grade materials. An other disadvantage of using dissimilar materials for the various parts of the diaphragm, namely, the partition or nozzle and the web and rim, stems from the fact that the plain carbon steel, ordinary gray cast iron or similar material used for the rim and web has a much higher carbon content than the partition. As a result, under operating conditions, there tends to be a certain amount of migration of the carbon from the plain carbon steel or gray cast iron to the adjacent portion of the partition, creating an abnormal brittleness in that part of the partition to which it migrates. This could result in cracking of the partition under operating conditions.

While any of a number of corrosion resistant and erosion resistant materials can be used for the diaphragms of steam turbines, cost considerations as well as the abovementioned problem of carbon migration dictate that an economical alloy be provided which is compatible with the partition material. At the same time, the material must be readily weldable, be castable, and have other desirable mechanical properties, such as weldability strength, and the like.

An object of this invention is to provide a material suitable for steam turbine diaphragms which has superior resistance to corrosion and erosion by steam.

A further object of the invention is to provide such a material which is similar to that of the partition material, so that carbon diifusion between the rim or web of the diaphragm and the partition will be eliminated.

A still further object of the invention is to provide an economical alloy as above, which is characterized by the fact that the only heat treatment necessary to develop desirable mechanical properties is a stress relief anneal after casting rather than the usual austenitizing, quenching and tempering.

Briefly, the invention comprises alloys and diaphragms made therefrom, said alloy containing, by weight, from 0.04 to 0.06% carbon, from 10 to 12% chromium, from 1.5 to 2.0% nickel, from 1.5 to 2.0% manganese, from 0.30 to 0.50% silicon, and a maximum of about 0.05% vanadium and about 0.10% maximum molybdenum with the remainder essentially iron. Diaphragms or other structures made therefrom are given a stress relief anneal after casting.

Those features of the invention which are believed to be patentable are specifically set forth in the claims appended hereto. The invention, however, will be better understood and additional objects and advantages thereof be appreciated from a consideration of the following description, and the drawing in which the single figure is a plan View of a typical steam turbine diaphragm which may be made in conjunction with the invention.

The alloys of the invention embody chemically balanced combinations of materials which provide superior corrosion and erosion resistance. They are economically made and possess the advantage that the only heat treatment necessary to develop suitable mechanical qualities is a simple stress relief anneal after casting rather than the usual hardening procedures which latter tend to cause Warping, cracking and the like. The carbon content of 0.04 to 0.06%, by weight, represents at its lower limit the lowest practicable limit for carbon and its higher limit the quantity which affords a material which is not detrimentally affected in so far as its hardness is concerned. The limit of 0.06% maximum carbon results in a structure of low hardness martensite when the alloy is air-cooled after welding. The weld is then stress relieved as described below for the casting. An excess of carbon also reduces the corrosion and erosion resistance of the material. The 10% to 12% of chromium in the alloy imparts corrosion resistance to the alloy, and provides a martensitic structure which is desired as compared to the ferritic structure which is promoted if more than about 12% of chromium is used. On the other hand, less than about 10% of chromium detracts from the corrosion resistance of the alloy. The 1.5% to 2% of nickel used also promotes the development of a martensitic structure, Whereas less than about 1.5% of nickel results in the production of a ferritic material. Amounts of nickel in excess of about 2% disproportionately increase the cost of the material. The 1.5 to 2.0% of manganese, like the nickel content, provides a martensitic structure, whereas less than 1.5% tends toward a ferritic material. More than 2% of manganese again increases the cost of the alloy. The 0.30 to 0.50% of silicon in the alloy acts as a deoxidizer in a well-known manner. The 0.05 maximum of vanadium again keeps the structure of the alloy martensitic. More than this amount of vanadium produces a ferritic structure and increases the high temperature strength so that the alloy is too resistant to the tempering which occurs during the stress relief anneal. The 0.10% maximum molybdenum, like the vanadium, keeps the structure of the alloy in a martensitic condition; more molybdenum tends toward a ferritic structure. Low molybdenum limits the resistance to tempering during the stress relief anneal.

Castings made from the above alloy are stress relieved for about 8 hours at 1200 B, it being realized that this stress relief is a time-temperature phenomenon and that the stress relief may be made at suitably lower tempera tures for longer periods of time and at suitably higher temperatures for shorter periods of time. It is recognized that this stress relief is the tempering of low hard. ness martensite.

Shown in the single figure of the drawing is a plan view of a half-segment of a diaphragm for a steam turbine. Such diaphragms are a typically useful application of the alloys of this invention, it being realized that such alloys may be used as well wherever a corrosion-resistant alloy which has desirable hardness after a simple stress relief, good weldability, and chipping and machining qualities, is desired. The diaphragm 1 consists of a rim section 2 and a web or hub section 3, between which are welded or cast in partitions or fixed nozzles 4. The relative thicknesses of the rims and hubs of diaphragms in various locations within a steam turbine will, of course, depend on the pressure stage in which the diaphragm is located.

The following examples will illustrate specific alloys which were cast according to this invention in the form of stearn turbine diaphragms.

EXAMPLE 1 An alloy was prepared and poured at 2935 P. which, in the as-cast condition, had a composition of, by weight, 0.04% carbon, 10.5% chromium, 1.49% manganese, 1.15% nickel, 0.30% silicon, 0.03% molybdenum, and 0.01% vanadium with the remainder essentially iron. The partitions were cast in place in the diaphragm and had a chromium content of about 12% and a carbon content which compared with that of the above alloy. The casting was stress relieved for 8 hours at 1200 F.

EXAMPLE 2 A second alloy was cast as above at a pouring temperature of 3000 F., this alloy having a composition of, by weight, 0.04% carbon, 11.1% chromium, 1.54% manganese, 1.16% nickel, 0.33% silicon, 0.02% molybdenum, and 0.01% vanadium with the remainder essentially iron. The partitions as above were cast in the diaphragm and the casting stress relieved for 8 hours at 1200 F.

EXAMPLE 3 Another alloy was prepared and poured at a temperature of 2950 C., which had an as-cast composition of, by weight, 0.04% carbon, 10.9% chromium, 1.58% manganese, 1.15 nickel, 0.35% silicon, 0.03% molybdenum, and 0.01% vanadium with the remainder essentially iron. The diaphragm cast as above with cast-in partition was stress relieved for 8 hours at 1200 F.

The above cast diaphragms were found to be characterized by good weldability and their chipping characteristics were comparable to those of the usual steel diaphragms. The machining qualities of the alloys were also good.

After stress relief, the hardness of the present alloys is in a useful range. For example, the as-cast 3,000 kg. Brinell hardness of the alloy of Example 1 was 331, and the stress relieved hardness was 192. For the alloy of Example 2, the as-cast hardness was 324 and the stress relieved hardness was 180. For the alloy of Example 3, the corresponding hardnesses were 320 and 181.

The room temperature, tensile and impact properties of the alloys of the invention are also of a deisrable nature.

Shown in Table 1 below are the tensile strength, yield strength, proportional limit, elongation, reduction in area, and Charpy test results for the three alloys.

Table 1 V-Notch Charpy (ft.'lbs.)

Tensile Strength p (p Reduction in Area,

Percent Proportional Limit mass:

By this invention, there are provided new and useful alloys which have desirable cororsion and erosion resistant properties, as well as desirable mechanical properties which can be arrived at without elaborate heattreating procedures.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A corrosion and erosion resistant alloy comprising, by weight, from about 0.04 to 0.06% carbon, from 10 to 12% chromium, from 1.5 to 2%nickel, from 1.5 to 2.0% manganese, from 0.30 to 0.50% silicon, a maximum of 0.05% vanadium, and a maximum of 0.10% molybdenum with the remainder essentially iron, said article being stress relieved.

2. A cororsion and erosion resistant alloy comprising, by weight, about 0.04% carbon, 10.5% chromium, 1.49% manganese, 1.15% nickel, 0.30% silicon, 0.03% molybdenum, and 0.01% vanadium with the remainder essentially iron.

3. A corrosion and erosion resistant alloy comprising, by weight, 0.04% carbon, 11.1% chromium, 1.54% manganese, 1.16% nickel, 0.33% silicon, 0.02% molybdenum, and 0.01% vanadium with the remainder essentially iron.

4. A corrosion and erosion resistant alloy comprising, by weight, 0.04% carbon, 10.9% chromium, 1.58% manganese, 1.15 nickel, 0.35% silicon, 0.03% molybdenum, and 0.01 vanadium with the remainder essentially iron.

5. The process of making a diaphragm for a steam turbine which comprises casting the rim and web sections of said diaphragm from an alloy comprising, by weight, from about 0.04 to 0.06% carbon, from 10 to 12% chromium, from 1.5 to 2% nickel, from 1.5 to 2% manganese, from 0.30 to 0.50% silicon, a maximum of 0.05% vanadium, and a maximum of 0.10% molybdenum with the remainder essentially iron, and stress-relieveing the resulting casting.

6. A stress relieved cast diaphragm for a steam turbine comprising, by weight, from about 0.04 to 0.06% carbon, from 10 to 12% chromium, from 1.5 to 2% nickel, from 1.5 to 2% manganese, from 0.30 to 0.50% silicon, a maximum of 0.05% vanadium, and a maximum of 0.10% molybdenum, with the remainder essentially iron.

7. A stress relieved cast article of manufacture comprising, by weight, from about 0.04 to 0.06% carbon, from 10 to 12% chromium, from 1.5 to 2% nickel, from 1.5 to 2% manganese, from 0.30 to 0.50% silicon, a maximum of 0.05% vanadium, and a maximum of 0.10% molybdenum, with the remainder essentially iron.

8. The process of making an article of manufacture which comprises casting said article from an alloy comprising, by weight, from about 0.04 to 0.06% carbon, from 10 to 12% chromium, from 1.5 to 2% nickel, from 1.5 to 2% manganese, from 0.30 to 0.50% silicon, a maximum of 0.05 vanadium, and a maximum of 0.10% molybdenum, with the remainder essentially iron and stress relieving said article.

References Cited in the file of this patent UNITED STATES PATENTS Sheridan et a1 Nov. 23, 1954 Kirkby et al. May 29, 1956 OTHER REFERENCES 

1. A CORROSION AND EROSION RESISTANT ALLOY COMPRISING BY WEIGHT, FROM ABOUT 0.04 TO 0.06% CARBON, FROM 10 TO 12% CHROMIUM, FORM 1.5 TO 2% NICKEL, FROM 1.5 TO 2.0% MANGANESE, FROM 0.30 TO 0.50 % SILICON, A MAXIMUM OF 0.05% VANADIUM, AND A MAXIMUM OF 0.10% MOLYBDENUM WITH THE REMAINDER ESSENTIALLY IRON, AND ARTICLE BEING STRESS RELIEVED. 